This study is designed to delineate specific risk factors which will differentiate SIDS victims from three different groups of control infants. To accomplish this, anatomical, behavioral, medical, familial and altitude variables are being investigated. (1) Detailed postmortem examinations which include xeroradiography and morphometry of the pharyngeal area with neck in flexed, neutral and hyperextended positions are performed on SIDS victims and control infants between the ages of 21 and 365 days who die of other causes. (2) In-depth interviews are conducted with both parents of the study child to answer 117 items on a questionnaire designed to elicit data about sleep disorders, medical and developmental histories and behavior. Also, genetic pedigrees which span at least three generations are constructed. Data provided by the parents is checked against medical and hospital records. SIDS families are compared with two groups of control families: (1) those with a child who, as a neonate, was hospitalized in a Newborn Intensive Care Unit for respiratory difficulties; and (2) those with a child who was born at the same hospital as a SIDS victim and who is matched for date of birth, gestational age, weight and sex. This clinical investigation will provide a large, computerized matrix of clinical, anatomical and behavioral data on well documented cases which will be available for additional future research.